The present invention relates to scanning probe microscopy, and in particular to the construction of a microscope and force-sensing probes in the form of cantilevers for use in atomic force microscopy.
In the conventional atomic force microscope (AFM), the deflection of a flexible cantilever is used to monitor the interaction between a probe-tip and a surface under study. As the tip is brought close to the surface, it deflects in response to interactions with the surface under study. These deflections are used to control the distance of the tip from the surface and to measure details of the surface. It is often desirable to operate an atomic force microscope in an oscillating mode. In this mode (known as AC mode), the cantilever is vibrated at a high frequency, and the change in amplitude (or phase) of the cantilever as it approaches a surface is used to control the microscope.
One reason for doing this is because, when oscillated at high amplitude, the probe is less likely to stick to the surface under study. However, this AC mode of operation is also intrinsically more sensitive. AC detection shifts the signal to be detected to sidebands on a carrier signal, avoiding the low frequency noise that DC signals suffer from. In addition, the mechanical Q of a cantilever resonance can be used to enhance the overall signal to noise ratio of a microscope operated this way.
In one version of the AC AFM as taught by Elings et al, U.S. Pat. Nos. 5,412,980 and 5,519,212, the oscillation is used mainly as a means of avoiding the effects of adhesion between the tip and surface. However, such adhesion is easily avoided by chemical means. For example, the microscope may be operated in a fluid which minimizes adhesion. Alternatively (or additionally), a tip material can be chosen so as to minimize its adhesion to the surface under study. In that case, there is no reason to operate the microscope at a large amplitude of oscillation.
The usual method of exciting motion in the AFM cantilever is to drive it with an acoustic excitation. This method works well in air or gas and has been made to work with the tip submerged in water as taught by Hansma et al, "Tapping Mode Atomic Force Microscopy in Liquids," Appl. Phys. Lett. 64: 1738-1740 (1994) and Putman et al, "Tapping Mode Atomic Force Microscopy in Liquid," Appl. Phys. Lett. 64: 2454-2456 (1994). However, in a fluid, the motions of the cantilever become viscously damped, so that substantial acoustic amplitude is required to drive motion of the cantilever. Furthermore, the fluid acts as a coupling medium between the source of acoustic excitation and parts of the microscope other than the cantilever. The result is that parts of the microscope other than the cantilever get excited by the acoustic signal used to vibrate the cantilever. If these motions lead to a signal in the detector, a background signal is generated which is spurious and not sensitive to the interaction between the tip and surface.
A scheme for exciting the cantilever directly has been described by Lindsay et al, "Scanning Tunneling Microscopy and Atomic Force Microscopy Studies of Biomaterials at a Liquid-Solid Interface," J. Vac. Sci. Technol. 11: 808-815 (1993). In this approach, a magnetic particle or film is attached to the cantilever and a solenoid near the cantilever is used to generate a magnetic force on the cantilever. This arrangement gives extreme sensitivity to surface forces, presumably because of a lack of background spurious signal as would occur in an acoustically-excited microscope. Lindsay, U.S. Pat. Nos. 5,515,719 and 5,513,518, the disclosures of which are hereby incorporated by reference, teach this novel form of AC-AFM in which the cantilever is excited by magnetic means.
Magnetic cantilevers are required in order to operate such a microscope. In the prior art at least three approaches were used. Lindsay et al, J. Vac. Sci. Technol. 11: 808-815 (1993), described a method for fixing a magnetic particle onto the cantilever. However, this method is not suitable for the fabrication of suitable cantilevers in quantity. O'Shea et al, "Atomic Force Microscopy of Local Compliance at Solid-Liquid Interfaces," Chem. Phys. Lett. 223: 336-340 (1994), describe a method for evaporating a magnetic coating onto the cantilevers. In order to avoid bending the cantilevers owing to the interfacial stress introduced by the evaporated film, they place a mask over most of the cantilever so that the magnetic film is deposited only onto the tip of the force-sensing cantilever. This approach requires precision alignment of a mechanical mask and it is not conducive to simple fabrication of suitable coated cantilevers. Other methods for the formation of magnetic films on cantilevers and for calibrating the properties of the films have been described in Lindsay, U.S. Pat. No. 5,612,491, and Han et al, U.S. Pat. No. 5,866,805, the disclosures of which are hereby incorporated by reference. Similar procedures have also been described by Cleveland et al, U.S. Pat. No. 5,670,712.
These references teach that a cantilever with a magnetic film or particle is deflected by the effect of forces that arise from the interaction of an applied magnetic field and a magnetic moment fixed to the cantilever. One embodiment of this approach, Lindsay, U.S. Pat. No. 5,515,719, is illustrated in FIG. 1. There, a magnetic particle or film 1 is attached to the cantilever 2 and magnetized so that its magnetic moment, M, 3 points away from the soft axis of the cantilever. A magnetic field gradient, dB/dz 4 is applied parallel to the magnetic moment on the cantilever tip 3, resulting in a force on the tip given by EQU F=M.times.dB/dz, (1)
where M is the magnetic moment and the magnetic field gradient, dB/dz, is applied along the same direction as the magnetic moment, resulting in a force, F. The generation of forces adequate to displace cantilevers of stiffness on the order of a Newton per meter by several nanometers requires either a very large magnetic moment or a very large field gradient.
Another prior art procedure as taught in Lindsay, U.S. Pat. No. 5,612,491, Han et. al, U.S. Pat. No. 5,866,805, and Cleveland et al, U.S. Pat. No. 5,670,712, is illustrated in FIG. 2. There, a film or particle 5 is magnetized so that its moment, M, 6, points along the soft axis of the cantilever 2. A magnetic field, B, 7, is directed perpendicular to the magnetic moment on the cantilever 6. This results in a torque, N, on the cantilever given by the equation EQU N=M.times.B (2)
This is roughly equivalent to a force F (8) on the end of the cantilever, given by the equation EQU F.apprxeq.N/L (3)
where L is the length of the cantilever. This effect has been demonstrated by Han et al, "A Magnetically Driven Oscillating Probe Microscope for Operation in Liquids," Appl. Phys. Left. 69, 4111-4113 (1996) who measured a motion of a few nanometers for an applied field of a few Oersteds, using a cantilever of stiffness 0.12 Newtons/meter.
The prior art procedures discussed above require a substantial magnetic moment to be affixed to the tip of the cantilever which limits the range of materials that may be used. In particular, iron alloys which have been used in the past oxidize easily, limiting the operation of the microscope to non-oxidizing environments. Accordingly, there is still a need in this art for a more sensitive method to create magnetic deflection of scanning probe microscope cantilevers and for materials which are more resistant to corrosion than iron-containing alloys.